title: Centrifugal analysis with automated sequential reagent addition: measurement of serum calcium.
abstract: We describe an automated method for calcium assay, for use with the Cobas-Bio centrifugal analyzer. Calcium is reacted with cresolphthalein complexone and the absorbance of the calcium--dye complex at 575 nm is measured. EDTA is then added to break up the calcium--dye complex and the absorbance at 575 nm is re-measured, to correct for endogenous color and turbidity. Day-to-day precision data, determined over four months, were as follows: mean = 92.9 mg/L, CV = 1.47%; n = 216; mean = 128.7 mg/L, CV = 1.72%; n = 216. Comparison of the Cobas-Bio method (y) with an atomic absorption spectrometric method (x) gave the following results: y = 1.012x--2.05, r = 0.991, Sy/x = 1.2, mean x = 92.63 mg/L, mean y = 91.69 mg/L, n = 74. Hemoglobin, bilirubin, or turbidity does not interfere. At the medical decision value (110 mg/L), the overall analytical error is 4.6 mg/L, which is less than the 5 mg/L allowable (95% confidence limit) error.
mesh_terms: D001322:Autoanalysis / Q000379:methods; D002118:Calcium / Q000097:blood*; D002498:Centrifugation; D002614:Chelating Agents; D004492:Edetic Acid; D006801:Humans; D009391:Nephelometry and Turbidimetry; D010635:Phenolphthaleins; D013053:Spectrophotometry; D013054:Spectrophotometry, Atomic
chemical_list: D002614:Chelating Agents; D010635:Phenolphthaleins; D004492:Edetic Acid; C017845:2-cresolphthalexon; D002118:Calcium